This invention relates to a moisture vapor permeable, substantially liquid impermeable composite sheet structure useful in apparel, surgical drapes, sterile wraps, packaging materials, protective covers, construction materials, and personal care absorbent articles such as diapers and sanitary napkins. More particularly, the invention is directed to a thin moisture vapor permeable film and a multiple layer fibrous substrate that combine to form a composite sheet that is durable, strong, and flexible, that acts as a barrier to liquids, bacteria, viruses and odors, and yet is also highly permeable to moisture vapor.
Various woven and nonwoven sheet materials used in making medical drapes, medical gowns and absorbent articles, such as diapers and sanitary napkins, must be comfortable, soft, pliable and substantially liquid impermeable. The sheet materials used in medical apparel and absorbent articles function to contain the discharged materials and/or to isolate these materials from the body of the wearer or from the wearer""s garments and bed clothing. As used herein, the term xe2x80x9cabsorbent articlexe2x80x9d refers to devices which absorb and contain body exudates, and, more specifically, refers to devices which are placed against or in proximity to the body of the wearer to absorb and contain the various exudates discharged from the body. Absorbent articles include disposable diapers, incontinence briefs, incontinence undergarments, incontinence pads, feminine hygiene garments, training pants, pull-on garments, and the like.
An ideal sheet material for use in medical apparel and absorbent articles will exhibit a high moisture vapor transmission rate that will reduce the build up of heat and humidity inside garments and articles made from the material. The ideal sheet material will also exhibit excellent barrier properties so as to prevent the passage or seepage of fluids, and will even prevent the passage of bacteria and viruses. The ideal material must also be strong enough so that it does not rip or delaminate under normal usage conditions regardless of whether the material is dry or wet. Where the sheet material is to be used in apparel, it is also important that the material be flexible, soft and drapable. Finally, where the sheet material is to be used in medical apparel, it is important that the sheet not generate fiber lint that might contaminate a medical environment.
PCT Publication No. WO 97/45259, which is hereby incorporated by reference, discloses a breathable composite sheet material comprised of a moisture vapor permeable thermoplastic film adhered to a fibrous substrate. The breathable thermoplastic film is primarily comprised of a polymer material selected from the group of block copolyether esters, block copolyether amides and polyurethanes. The fibrous substrate is a nonwoven sheet made primarily of a polymer fibers that are not compatible with the film, such as a polyolefin fibers. The film is adhered to the fibrous substrate by extruding a layer of the molten film-forming polymer directly onto the fibrous substrate and then mechanically engaging the film and the fibers of the substrate, as for example by pressing the molten film into the fibrous substrate in a nip formed between two rolls.
U.S. Pat. No. 5,445,874 discloses a waterproof, blood-proof and virus-proof laminate material suitable for use in protective apparel. The laminate is comprised of a moisture vapor permeable film adhered to a woven or nonwoven fabric. The preferred film is a thermoplastic polyester elastomer. The disclosed fabrics include nonwoven fabrics of polyester, nylon and polypropylene. U.S. Pat. No. 5,445,874 discloses that the film can be laminated to the fabric by powder adhesive lamination, hot melt lamination, or wet adhesive lamination.
Adhesive lamination, thermal lamination and extrusion coating methods have all been used to produce composite sheets of a fibrous nonwoven substrate and a moisture vapor permeable, substantially liquid impermeable film. It has been possible to make such composite sheets with good barrier properties so long as the moisture vapor permeable film is relatively thick (i.e.,  greater than 25 microns). However, it has been difficult to make such composite sheets with thinner films without sacrificing important barrier properties. Very thin moisture vapor permeable films are desirable in a composite sheet because thinner films facilitate greater flux of moisture vapor through the composite sheet and because thinner films use less of the film material and are accordingly less expensive to produce.
Adhesive lamination is carried out in a post film formation step. For adhesive lamination to be feasible, the moisture vapor permeable film must have enough tensile strength and tear strength so that the film can be formed, wound onto a roll, and later unwound and handled during the adhesive lamination process. It is difficult to handle moisture vapor permeable films less than 25 microns (1 mil) in thickness during the adhesive lamination process without tearing the film or introducing defects into the film.
Thermal lamination of moisture vapor permeable films less than 25 microns thick has similarly resulted in composite sheet materials with inadequate barrier properties. When composite sheets are made by thermally laminating a thin film to a fibrous substrate, the thin film handling problems associated with adhesive lamination, as described above, are encountered. In addition, to carry out a thermal lamination, the film must be subjected to elevated temperatures and pressures so as to soften the film and force it into mechanical engagement with the fibrous substrate. Generally, the peel strength between the film and the fibrous substrate increases with increasing lamination temperatures and increasing nip pressures. Unfortunately, when moisture vapor permeable films with a thickness of less than 25 microns are subjected to the increased temperatures and pressures needed to obtain adequate peel strength in the composite sheet, small holes develop in the film such that the composite sheet does not exhibit the fluid barrier properties desired in a composite sheet for use in absorbent articles or medical apparel. These defects can result from non-uniform temperatures throughout the web during bonding or from high nip pressures.
A composite sheet with excellent tensile and peel strength, that does not emit loose fibers, can be produced using a carded web of staple fiber that is powder bonded with an adhesive that is compatible with the fibers of the web. The composite sheet is produced by extrusion coating the powder-bonded web with a molten thin film that is also compatible with the fibers of the web and the powder adhesive. xe2x80x9cCompatibilityxe2x80x9d of thermoplastic materials is an art-recognized term that refers, generally, to the degree to which the thermoplastic materials are miscible with each other and/or interact with each other. xe2x80x9cIncompatiblexe2x80x9d materials, as used herein, means materials that are substantially immiscible with each other or do not interact with each other. Incompatible materials do not wet or adhere well to each other, even when heated. As used herein, xe2x80x9ccompatiblexe2x80x9d materials are materials that are not xe2x80x9cincompatiblexe2x80x9d with each other, as defined above. For purposes of this application, a fiber is deemed to be compatible with a synthetic adhesive or with another polymer if the adhesive or other polymer is miscible with material that comprises the majority of the fiber and if the adhesive or other polymer readily wets the fiber or if the adhesive or polymer can adhere well to the fiber.
A composite sheet, made from a powder-bonded web that has been extrusion coated with a film of a thermoplastic polymer compatible with the fibers of the web and the solidified powder adhesive, exhibits good tensile strength and low linting because the solidified powder adhesive binds all of the fibers in the web into a strong matrix. These sheets exhibit excellent peel strength because the film readily adheres to the compatible adhesive and fibers of the web. For example, excellent tensile strength, peel strength and linting resistance can be obtained where the film, the nonwoven and the adhesive are all comprised of polyester polymers. Unfortunately, the film layer in composite sheets of this type is so thoroughly and completely bonded to the nonwoven that the sheet has a stiff paper-like feel that is unsuitable for apparel or many kinds of absorbent articles.
Accordingly, there is a need for a composite sheet material that acts as a barrier to fluids, bacteria and viruses, yet is also highly permeable to moisture vapor. Such a moisture vapor permeable, fluid impermeable composite sheet material should be durable, strong, and low linting, while at the same time being soft, flexible and comfortable enough for use in apparel products and absorbent articles. There is a further need for such a composite sheet that can be produced in an economical fashion, i.e., film extrusion and lamination in one process.
The invention provides a moisture vapor permeable, substantially liquid impermeable composite sheet material. The sheet material comprises a first fibrous nonwoven web having a first side and an opposite second side, and a second fibrous nonwoven web having a first side and an opposite second side. The first side of the second fibrous nonwoven web abuts the second side of the first fibrous nonwoven web, and the first and second fibrous nonwoven webs each are powder-bonded webs wherein the fibers of the first and second fibrous webs are bonded to the other fibers of such web by a synthetic adhesive permeating the first and second nonwoven fibrous webs. The first and second fibrous nonwoven webs are bonded to each other by the adhesive. A moisture vapor permeable thermoplastic film is bonded to the second side of the second fibrous nonwoven web. At least 90 weight percent of the fibers in the first fibrous nonwoven web are compatible with the adhesive, between 25 and 75 weight percent of the fibers in the second fibrous nonwoven web are compatible with the adhesive and the thermoplastic film, and between 75 and 25 weight percent of the fibers in the second fibrous nonwoven web are incompatible with the adhesive and the thermoplastic film. At least 50 weight percent of the polymer in the thermoplastic film is also compatible with the adhesive.
Preferably, the weight of the fibers in the second nonwoven fibrous web is between xc2xc and 4 times the weight of the fibers in the first nonwoven fibrous web. It is also preferred that the film of the composite sheet have an average thickness of less than 25 microns, and more preferably less than 20 microns. The composite sheet ideally exhibits a peel strength of at least 0.1 N/cm, a hydrostatic head of at least 60 cm, and a moisture vapor transmission rate, according to the LYSSY method, of at least 1000 g/m2/24 hr.
According to a preferred embodiment of the invention, the adhesive in the nonwoven web is a polyester polymer or polyester copolymer adhesive, and the moisture vapor permeable film is comprised of at least about 75% by weight of polymer selected from the group of block copolyether esters, block copolyether amides, copolyether imide esters, polyurethanes, polyvinyl alcohol, and combinations thereof. In the preferred embodiment, at least 90 weight percent of the fibers in the first fibrous nonwoven web are made of polymer selected from the group of polyester polymers and copolymers, between 25 and 75 weight percent of the fibers in the second fibrous nonwoven web are made of polymer selected from the group of polyester polymers and copolymers, and between 75 and 25 weight percent of the fibers in the second fibrous nonwoven web are made of polymer selected from the group of polyamides, polyolefins, acrylics, and cotton. The polyester polymers and polyester copolymers in the fibers of the preferred embodiment are preferably selected from the group of poly(ethylene terephthalate), poly(1,3-propylene terephthalate) and copolymers thereof. At least 10% of such polyester fibers may be shaped fibers having a scalloped-oval cross-section. According to a preferred embodiment of the invention, the moisture vapor permeable film is comprised of at least about 75% by weight of block copolyether esters, and more preferably the film consists essentially of a copolyether ester elastomer.
The composite sheet of the invention is substantially free of pinholes, and substantially no liquid passes through the sheet when tested according to the liquid seepage test. It is further preferred that the composite sheet prevent the passage of microbes when tested according to the ISO 11607 standard for sterile packaging materials and that the composite sheet prevents the passage of microbes and viruses with a diameter greater than 0.025 microns when tested according to ASTM F1671.
The moisture vapor permeable film of the composite sheet of the invention may have first and second layers, each of which are comprised of a different moisture vapor permeable thermoplastic polymer composition. The first layer of such a moisture vapor permeable film may comprise at least 75% of the total weight of the film and may comprise a substantially hydrophilic layer, while the second layer of the moisture vapor permeable film may comprise a substantially hydrophobic layer, wherein the first layer of the moisture vapor permeable film is bonded to the second side of the second fibrous nonwoven web.
The present invention is also directed to an item of apparel or a protective cover comprising the composite sheet material described above.
The present invention also includes a method for making a moisture vapor permeable, substantially liquid impermeable composite sheet comprising a fibrous nonwoven bonded with a powder adhesive and adhered to a moisture vapor permeable thermoplastic film. The method includes the steps of: (a) providing a first fibrous nonwoven web having a first side and an opposite second side, at least 90 weight percent of the fibers in the first fibrous nonwoven web being compatible with a powder adhesive; (b) providing a second fibrous nonwoven web having a first side and an opposite second side, and abutting the first side of the second fibrous nonwoven web with the second side of the first fibrous nonwoven web, between 25 and 75 weight percent of the fibers in the second fibrous nonwoven web being compatible with the adhesive and the thermoplastic film, and between 75 and 25 weight percent of the fibers in the second fibrous nonwoven web being incompatible with the adhesive and the thermoplastic film; (c) permeating the adhesive throughout the first and second fibrous nonwoven webs; (d) heating the web to a temperature sufficient to melt the adhesive and powder-bond the webs in a manner such that the fibers of the first and second fibrous webs are bonded to the other fibers of such respective webs by the adhesive permeating the first and second nonwoven fibrous webs, and the first and second fibrous nonwoven webs are bonded to each other by the adhesive; (e) melt extruding the moisture vapor permeable thermoplastic film onto the second side of the second fibrous nonwoven web to form composite sheet; (f) subjecting the composite sheet material to a confining pressure by passing the composite sheet material through a nip; and (g) collecting the composite sheet onto a roll.